The BBC's funding is increasingly under threat and the license fee is being used to fund a wide range of non-core and non-BBC services; mandatory logins are a hedge against several risks.
Over-75s are entitled to a free TV license; this was historically funded by government subsidy, but that subsidy will be completely phased out by 2020-21. This will create a revenue shortfall of about £0.5bn, of a total budget of around £4bn.
On the other end of the age spectrum, an increasing proportion of young people are living in TV-less households, making it far easier for them to continue watching BBC programming while evading the license fee.
The BBC is increasingly reliant on international commercial partnerships (most notably the BritBox streaming service) in order to raise revenues, but this is potentially jeopardised by the increasing use of VPN services; GeoIP walls aren't very useful if a large proportion of internet users can choose their IP location with a couple of clicks.
Mandatory logins make a lot of possible future scenarios easier. The BBC may decide to tie logins to the license fee if rates of digitally-assisted fee evasion become untenable. The license fee may be abolished altogether by a future Conservative government, necessitating a shift to a subscription model. The BBC might also decide to open up iPlayer and Sounds to an international audience on a subscription basis.
If you object to mandatory logins, I'd suggest writing to your MP to ask them to help safeguard the BBC's funding.